The first phase will be conducted in a randomized, placebo-controlled, double-blind manner to evaluate the efficacy of fluvoxamine on change from baseline to the last observation visit in the JCY-BOCS 10-item total score. Eligible patients will be allocated to the fluvoxamine group or placebo group in a 1:1 ratio using the experience of fluvoxamine treatment and age as stratification factors (dynamic allocation). The first phase consists of a screening period of 1-2 weeks, a forced titration dose period of 2 weeks, a dose adjustment period of 4 weeks, a maintained dose period of 4 weeks, and a tapering dose period of 0-4 weeks.